Atlantic Sea Ice Could Grow in the Next Decade

[LouisvilleDem]

https://eos.org/research-spotlights/atlantic-sea-ice-could-grow-in-the-next-decade

Apparently ice extents are controlled more by ocean currents than global temperatures.

The massive conveyor belts of the ocean and atmosphere transfer energy around the globe and drive Earth’s climate. Improved models and increases in computer power are starting to allow scientists to get a better glimpse of future surface conditions in the Atlantic by taking into account changes in the ocean heat conveyor. The ocean’s influence on sea ice is not obvious, but in a new study, Yeager et al. argue that it plays a key role in accurate projections of sea ice.

The researchers analyzed simulations from the Community Earth System Model, modeling both atmosphere and ocean circulation. They found that decadal-scale trends in Arctic winter sea ice extent are largely explained by changes in ocean circulation rather than by large-scale external factors like anthropogenic warming.

The team emphasized the influence of the thermohaline circulation (THC), a global current that carries heat around the planet and that experts believe has been slowing down in the Atlantic since about 2000. Although anthropogenic warming may produce a long-term global temperature rise, the THC slowdown contributes to short-term cooling in the subpolar Atlantic and, consequently, a decline in the ice melt rate. The researchers make the connection between these circulation changes and satellite observations taken between 2005 and 2015 that show a positive trend in winter ice cover. In other words, slowing circulation hinders heat transport to the North Atlantic, allowing surface waters to stay cool and sea ice to expand.

Ultimately, the rise of global temperatures will generate a loss of sea ice cover over the coming century. This study is a stepping-stone toward the ultimate goal of decadal climate prediction, which is vital to understanding and anticipating the short-term trends and changes that communities will be tackling in the near future.

[NickB79]

1. And what is causing the thermohaline current to slow? Climate change

The team emphasized the influence of the thermohaline circulation (THC), a global current that carries heat around the planet and that experts believe has been slowing down in the Atlantic since about 2000. Although anthropogenic warming may produce a long-term global temperature rise, the THC slowdown contributes to short-term cooling in the subpolar Atlantic and, consequently, a decline in the ice melt rate. The researchers make the connection between these circulation changes and satellite observations taken between 2005 and 2015 that show a positive trend in winter ice cover. In other words, slowing circulation hinders heat transport to the North Atlantic, allowing surface waters to stay cool and sea ice to expand.

The cause of the THC slowdown? Massive freshwater pulses surging out from melting glaciers and polar ice: https://www.washingtonpost.com/news/energy-environment/wp/2015/03/23/global-warming-is-now-slowing-down-the-circulation-of-the-oceans-with-potentially-dire-consequences/

Nonetheless, the new research finds that global warming does indeed seem to be slowing down the circulation. And while hardly catastrophic, that can’t be good news. Among the very real effects, notes the Potsdam Institute’s Rahmstorf, could be a possible increase in U.S. sea level if the whole circulation were to break down — which would be seriously bad news for cities like New York and Boston.

The study uses a reconstruction of sea surface temperatures in the North Atlantic to find that starting in around 1970 or 1975, the overturning circulation started to weaken — an event likely triggered by an unusual amount of sea ice traveling out of the Arctic ocean, melting, and causing freshening. The circulation then started to recover in the 1990s, but “it seems this was only a temporary recovery, and now it’s actually further weakened,” says Rahmstorf.

The hypothesized reason for further declines presented by the paper is that the massive Greenland ice sheet may now be losing enough freshwater due to melting to weaken the circulation. And indeed, it appears that a particular ocean region of the North Atlantic south of Greenland and between Canada and Britain is becoming colder — an indicator of less northward heat transport.

And even beyond that, your own article states this is a short-term, decadal variability within the system that will ultimately be overwhelmed by a warming climate in the decades to come.

[LouisvilleDem]

2. Maybe

At this point we don't have enough observations to say anything definitively. Our understanding of ocean currents is primitive at best, and we only have good data since the Argo array was deployed (first buoys launched around 2000, full coverage not available till 2004).